This invention relates to ion exchangers and, more particularly, this invention relates to inorganic anion exchangers.
U.S. Pat. No. 3,002,932, issued to E. J. Duwell and J. W. Shepard, describes several noncrystalline inorganic anion exchangers consisting essentially of hydrated oxides of pairs of elements selected from the group consisting of aluminum, silicon, titanium, zinc, and zirconium. The compositions above are prepared by coprecipitating the hydrates of the combination of the oxides of the elements referred to above in an aqueous medium. The coprecipitation is carried out by gradually adding a base to an acid below a pH of 5 to bring the aqueous solution to a pH of about 5 to 7. The aqueous mixture is dried below about 150.degree. C., followed by washing the dried mixed hydrated oxide with water, and again finally drying the mixture below about 150.degree. C.
In a paper by E. J. Duwell and J. W. Shepard, "The Preparation and Properties of Some Synthetic Inorganic Anion Exchangers", Journal of Physical Chemistry, Volume 63, December, pages 2044-47, (1959), various amorphous inorganic anion exchangers are described. The paper describes a method for preparing an amorphous Zn(OH).sub.2 exchanger and Al(OH).sub.3 exchanger. The amorphous exchangers are prepared by coprecipitating cations of higher valence with aluminum and zinc hydroxide in slightly acidic solutions to form a gel. The gels are then dried to form white, finely divided amorphous powders of Al(OH).sub.3 and Zn(OH).sub.2.
In the prior art above, it is reported that dehydration of the exchangers at 150.degree. C. results in the loss of anion exchanger capacity. Thus, the compositions prepared by the methods disclosed in the art cited above are unsatisfactory as anion exchangers at temperatures above 150.degree. C.
It is desired, therefore, to provide novel inorganic anion exchangers with increased thermal and chemical stability at elevated temperatures and with greater exchange capacity at elevated temperatures than previously known inorganic exchange materials. It is also desired to provide a method by which these anion exchangers may be prepared.